Extreme ultraviolet light, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates, e.g., silicon wafers.
Methods to produce a directed EUV light beam include, but are not necessarily limited to, converting a source material into a plasma state that has at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser-produced-plasma (“LPP”), the required plasma can be produced by irradiating a target material having the required line-emitting element, with a laser beam.
One particular LPP technique involves generating a stream of source material droplets and irradiating some or all of the droplets with laser light pulses, e.g., zero, one or more pre-pulse(s) followed by a main pulse. In more theoretical terms, LPP light sources generate EUV radiation by depositing laser energy into a source material having at least one EUV emitting element, creating a highly ionized plasma with electron temperatures of several 10's of eV. The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma in all directions. In one common arrangement, a near-normal-incidence mirror (often termed a “collector mirror”) is positioned at a relatively short distance, e.g., 10-50 cm, from the plasma to collect, direct (and in some arrangements, focus) the light to an intermediate location, e.g., a focal spot. The collected light may then be relayed from the intermediate location to a set of scanner optics and ultimately used to illuminate a resist-coated wafer. To efficiently reflect EUV light at near normal incidence, a mirror having a delicate and relatively expensive multi-layer coating is typically employed. Keeping the surface of the collector mirror clean and protecting the surface from plasma-generated debris has been one of the major challenges facing the EUV light source developers.
In quantitative terms, one arrangement that is currently being developed with the goal of producing about 100 W of EUV light at an intermediate location contemplates the use of a pulsed, focused 10-12 kW CO2 drive laser which is synchronized with a droplet generator to sequentially irradiate about 10,000-200,000 tin droplets per second. For this purpose, there is a need to produce a stable stream of relatively small droplets (e.g., 5-100 μm in diameter) at a relatively high repetition rate (e.g., 10-200 kHz or more) and deliver the droplets to an irradiation site with high accuracy and good repeatability in terms of timing and position over relatively long periods of time.
In this regard, U.S. patent application Ser. No. 12/214,736, filed on Jun. 19, 2008, entitled SYSTEMS AND METHODS FOR TARGET MATERIAL DELIVERY IN A LASER PRODUCED PLASMA EUV LIGHT SOURCE, discloses a droplet generating system in which a pusher gas forces a source material, e.g., molten tin, to flow from a reservoir under pressure through a glass capillary tube which has a relatively small diameter and a length of about 10 to 50 mm. As described therein, an electro-actuatable element such as a piezoelectric actuator may be coupled to the capillary tube to disturb the fluid flowing through the tube and create a stream of droplets. In this manner, the capillary tube acts as a restriction to the flow of source material into the plasma chamber. Unfortunately, the fragile glass capillary tubes are prone to failure, generally either by fracture, or in some cases, the high pressure may cause the capillary tube to be pushed out of its fitting (i.e., the joint where the capillary attaches to a metal reservoir. When this happens, a large amount of pressurized droplet material may be sprayed into the EUV light source vacuum chamber through the large opening that develops as a result of the failure. For example, if a 1 mm diameter hole were to develop, a 0.5 L source material reservoir holding liquid tin that is maintained at 1000 psi pressure could be emptied in as few as 18 seconds. Such an event may result in substantial contamination and damage to the EUV light collecting mirror, vacuum hardware, and/or the chamber's in-vacuum diagnostic devices. Furthermore, once the reservoir holding the source material, e.g. tin, is emptied, the vacuum chamber may be pressurized by the pusher gas and this pressurization may damage the chamber's vacuum pumps.
With the above in mind, applicants disclose systems for protecting an EUV light source chamber from high pressure source material leaks and corresponding methods of use.